The diagnosis and management of glaucoma and other disorders of the optic nerve require sensitive methods for detecting and measuring damage to the retinal nerve fibers. Measurement of visual sensitivity with static perimetry and ophthalmoscopic assessment of the condition of the optic disc are currently used in clinical practice, but newer methods for assessing the nerve fiber layer of the retina directly are being developed. All such methods depend on the interaction of the nerve fiber layer with the light used to observe it, but there is little quantitative data describing this interaction. Furthermore, the mechanisms by which light and the nerve fibers interact are unknown. The long-term objective of the research proposed in this application is to provide a comprehensive quantitative description of the optical properties of the nerve fiber layer of the retina, and to establish the anatomical basis for its reflectance. AChieving this objective will provide a solid foundation upon which all methods for assessing the nerve fiber layer can be developed, and the knowledge gained may lead to totally new approaches. The specific aims of this project are to 1) test the hypothesis that the major component of NFL reflectance (wavelengths shorter than 560 nm) arises from light scattered by thin, cylindrical organelles (perhaps microtubules) of nerve axons and that reflectance at long wavelengths (>560 nm) results from a second mechanism and 2) test the hypothesis that nerve fiber layer reflectance is proportional to nerve fiber layer thickness. Nerve fiber layer reflectance will be measured in vivo by directional imaging reflectometry and in vitro by imaging microreflectometry to determine if short-wavelength reflected light exhibits the optical properties expected of reflections from this cylinders and to see if a second spectral component can be revealed by various manipulations. Reflectometrically studied areas of nerve fiber layer will then undergo morphometric analyses by light and electron microscopy to determine the amount of nerve fiber tissue present and to quantify the cellular organelles (microtubules, neurofilaments, and cell membranes) in the nerve axons.